The invention relates to a divided flywheel having two flywheel elements arranged coaxially with each other. The flywheel masses are substantially distributed equally between the two elements and there is a spring arrangement connected therebetween. The spring arrangement has at least two groups of springs connected in parallel, one of which has play. A frictional coupling is also operative between the two flywheel elements.
In a divided flywheel of this type which is known from DE-OS 3,629,225, at least one part of the frictional coupling arranged between the flywheel elements is continuously operative. Another coupling part has play, that is to say it is inoperative within a certain angle of rotation. When this angle of rotation is used up by relative movement between the flywheel elements, the other coupling part also becomes operative. Then as soon as the direction of the relative movement of the flywheel elements is reversed, the total play of the other coupling has to be used up before it can also become operative in that direction of the relative movement.
This known divided flywheel still does not exhibit ideal behavior.
A divided flywheel should in principle operate in the so-called supercritical range, that is to say the frequency of the vibrations excited by the engine during road service should lie above the resonance frequency of the divided flywheel and simultaneously, a negligible frictional engagement should exist between the flywheel elements. If both conditions are ensured, then the transmission line and the engine are virtually completely decoupled vibrationally from each other. At low engine speeds, and particularly near the no-load speed of the engine, the operation of the divided flywheel in the supercritical range cannot immediately be ensured, because at these speeds the resonance frequency of the flywheel and the frequencies of the vibrations excited by the engine lie close together. They even coincide under some conditions. In this service phase, a high frictional engagement between the flywheel elements is desirable in order to prevent excessive vibration amplitudes occurring due to resonance effects.
With the flywheel known from DE-OS 3,629,225 it is impossible to ensure maximum comfort either at high speeds or at low speeds, because at high speeds, i.e., during normal road service of a motor vehicle, it is impossible to achieve the desirable complete vibrational decoupling of the engine and transmission line due to the one coupling which always remains operative.
It is therefore the object of this invention to produce a divided flywheel which permits optimum vibrational isolation between engine and transmission line under all service conditions, without permitting relative movements of the flywheel elements with excessive amplitudes during load changes or when passing through the resonance range.
This object is achieved in a flywheel of the type initially defined in that the frictional coupling has play (its friction elements or friction lamellae being connected with play in the circumferential direction to at least one flywheel element) and that the spring constants of the groups of springs are dimensioned so that the frequencies of the vibrations which can be generated at the starting speed and no-load speed of the engine lie above a low first critical frequency, determined by the group of springs without play, and the frequencies of the vibrations which can be generated at service speeds lie above a higher second critical frequency of the flywheel, determined by both groups of springs conjointly.
The invention utilizes the fact that an internal combustion engine exhibits an extremely low torque at low speeds and the spring arrangement between the flywheel elements therefore has to transmit only low torques if any. These low torques can be absorbed solely by the group of springs without play, without stressing the group of springs with play. The resonance frequency of the flywheel when transmitting extremely low torques is therefore determined substantially only by the group of springs without play, with the result that the resonance frequency has an extraordinarily low value. Therefore even vibrations excited at very low speeds, which have a very low frequency, may lie in the supercritical range of the flywheel. The flywheel according to the invention is therefore also suitable for modern engines which have a very low no-load speed.
As soon as higher torques have to be transmitted, the group of springs with play becomes operative in addition to the group of springs without play. Although this has the result that the resonance frequency of the flywheel is raised correspondingly, nevertheless, since the engine can generate higher torques only at higher speeds and the vibrations generated at higher speed have a higher frequency, the vibrations generated in the transmission line or by the engine remain in the supercritical range of the flywheel.
It is moreover advantageous that the two groups of springs can conjointly have greater rigidity, and accordingly the maximum relative rotation between the flywheel elements remains comparatively small. Undesirable resilience in the transmission line during load changes, and also undesirably high vibration amplitudes of the flywheel elements during the staring phase, are therefore prevented.
During normal service, i.e., so long as the flywheel operates in the supercritical range, the coupling with play virtually does not change the behavior of the flywheel, because the coupling is adjusted within a very short time under the influence of relative movements of the flywheel elements, so that the occurring vibrations have only small vibration amplitudes in the supercritical range and lie within the range of play.
On the other hand, the coupling with play operates in the sense of limitation of the vibration amplitudes, particularly during the starting phase, because when the play is used up, an additional resistance becomes operative between the flywheel elements in the sense of braking relative movements of the flywheel elements.
With a view to the smoothest possible operation of the flywheel, it is advantageous if the coupling play is somewhat greater than the play of the one group of springs.
It is also possible to have a further short-stroke rigid group of springs connected in parallel with a frictional coupling and arranged in series with the groups of springs which are connected in parallel. This further group of springs, conjointly with the friction coupling which is connected in parallel, damps excessive movements which the flywheel elements may occasionally execute under extremely unfavorable conditions or when the resonance frequency of the flywheel is excited for a long time during a plurality of attempts to start.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.